1. Field of the Invention
The present invention relates generally to solar collectors, and more particularly to a stretched membrane-type variable focusing collector and a method for manufacturing same.
2. Description of the Prior Art
Recent developments in the solar collector art include a trend toward manufacturing reflectors for concentrating solar collectors with thin flexible lightweight reflector materials. Examples of such lightweight reflector materials are thin metallic sheets of steel or aluminum which are often called foils. Collectors manufactured from these materials are commonly referred to as stretched membrane solar collectors. Generally speaking, a solar collector consists of a reflector and metal-constructed reflector support frame. The reflector is typically in the form of a mirror or plurality of mirror segments.
Individual solar collectors are frequently employed in an array to concentrate solar radiation severafold by reflecting and focusing the solar radiation onto an absorber/receiver. Solar radiation is commonly known as sunlight and, generally speaking, concerns electromagnetic radiation and particles emitted by the sun. The absorber/receiver which may be a cavity-type is positioned at a focal area of the array so as to absorb maximum heat energy.
The focal area, broadly speaking, concerns a point or region to which the collector reflects all incident sun radiation. The solar energy flux received and absorbed by the receiver/absorber is usually carried away by a suitable heat transfer fluid to either operate a thermodynamic heat engine or to provide process heat. Solar energy flux generally means energy flux transmitted from the sun, which is in the form of electromagnetic radiation.
The trend toward producing lightweight solar collectors is dictated in part by high manufacturing costs of glass/metal-type reflectors. This trend is also dictated in part by the heavyweight of glass/metal-type heliostat reflector panels. A heliostat may be simply defined as a tracking mirror. To continue, the reflector panels and support structure are often fabricated from thick heavy metal, glass and composite materials in order to have them meet strength and rigidity requirements imposed by the heliostat performance. Speaking more specifically, such strength and rigidity is frequently required in order to give the panel the capacity to withstand environmental loads without undergoing warping, buckling, or fracturing which eventually could lead to failure, as well as being required to maintain optical accuracy. Examples of such environmental loads are gravity loads, wind loads, and ice/snow loads.
Unfortunately, the heavy deadweight load of the reflector and the reflector support panel frequently produces stresses and deformations in the heliostat which undesirably add to the harmful stresses produced by environmental loads. Additionally, the use of heavy structural elements and metal materials to add sufficient strength and ridigity so that the heliostats can sustain such loads is one major reason for their undesirably high manufacturing costs.
In addressing the disadvantages associated with heavyweight collectors by producing collectors which employ substantially thinner and lighter weight materials, a problem has developed in shaping and tensioning the stretched reflector surface thereof. It has often been extremely difficult to shape and tension a stretched membrane-type reflector surface such that it produces an acceptable focal spot at the absorber/receiver cavity with minimal unabsorbed surface reflected solar flux and associated radiant and convection energy losses. Radiant and convection losses concern solar energy that is lost by the receiver/absorber after the solar radiation is absorbed. The concept of radiant and convection losses become even more significant when it is realized that the characteristics of a stretched reflector surface and a focus provided thereby may be utilized to reduce such losses.
With regard to such reflector characteristics and reflector focus, a stretched reflector surface will generally have a gravity induced focal length which is a function of a reflector elevation angle and a surface tension. Additionally, the focus of the collector depends to a large extent on sizing and centering the reflector surface with a certain surface configuration and a resulting surface angle defined thereby. Normally, increasing the tension of the stretched reflector surface increases the focal length. The ideal focal length is equal to a slant range from the reflector to the absorber/receiver cavity. The aforesaid characteristics of stretched reflector surfaces can be used to enhance collector system performance by reducing the size of the image at the receiver and therefore the amount of energy spillover.
Additionally, a problem has developed in providing lightweight collectors with variable or adjustable focusing capabilities such that the collector can be used to produce various concentration ratios to meet specific collector site requirements. Concentration ratios concern the ratio of an intensity of solar light impinging on the absorber/receiver to that of the solar light impinging on the collective surface of the collector. Notably, these ratios may be as small as one for no concentration to as high as several thousand. Yet another problem concerns producing a lightweight collector that is capable of withstanding wind induced deflections of the collector surface with minimal deterioration of the collector focus.
To cope with the aforesaid problems, the reflector surfaces of some solar collectors have been designed by tensioning a sheet of aluminized Mylar over a plurality of elongated supporting members. The supporting members function to impart a caternary configuration to the aluminized sheet. A prior art patent relating to such a design is U.S. Pat. No. 4,173,397. Unfortunately, however, this prior art design as well as others have suffered from one or more shortcomings. For example, this earlier design is unduly complex and comprises a number of component parts and its focus is not easily controllable.
Some prior art designs have stretched a sheet of aluminized Mylar over the top of a hollow cylinder and reduced the pressure therein between to provide a desired surface configuration. An example of this design is disclosed in U.S. Pat. No. 4,288,146. However, unfortunately, this design may result in a proneness to develop leaks and changes in the pressure within the cylinder. Such leaks may, in turn, lead to undesirable and irreversible degradation of the collector focus. It will be noted that the use of a vacuum pump to maintain the desired pressure has to some degree been partly helpful in reducing some aspects of the problem with pressure leakage. However, such a pump is an additional cost element and is power consuming.
Some prior art designs use flat surface-type collectors. In flat surface-type collectors the reflected sunlight radiation is aimed rather than focused at the absorber/receiver cavity. Flat surface-type collectors, however, when employed in applications where high intensity ratios are desired, often produce an unacceptably enlarged focal region at the receiver as a consequence of spreading of the reflected incident sunlight beam, as well as to produce a related unwanted drop in optical efficiency. Optical efficiency generally concerns a measurement of a fraction of the sun energy that actually reaches the absorber/receiver cavity.